Prior art imaging systems using penetrating radiant energy illustrate a variety of architectures to achieve, within the constraints of practical realities, the ability to produce an image which accurately portrays the object being imaged while minimizing the radiation dose required to achieve that image. Two important characteristics play a large part in determining the accuracy with which an image portrays the object being imaged, those characteristics are the contrast and resolution of the image. Resolution is, of course, important so that objects being imaged can be precisely located, or in the case of relatively small objects, they can be even seen at all. Contrast is significant so that different materials within the object being imaged are portrayed as different.
One form of prior art penetrating radiant energy imaging systems, which are generally characterized as low dose systems, form a pencil beam of penetrating radiant energy which repeatedly scans a line in space at which a radiant energy detector is located, and the object being imaged is located between the source of the pencil beam and the detector. A single sweep of the pencil beam across the detector is then capable of producing a signal representative of the intensity of the radiant energy reaching the detector which can be correlated with the density of the portion of the object illuminated by the penetrating radiant energy. By providing relative motion between the X-ray source-detector combination with respect to the object being illuminated, different sweeps of the pencil beam can be made to scan different sections of the object being illuminated so that, over time, a raster type sweep of the object being illuminated is effected. Typically, the detector comprises an X-ray to electrical signal transducer, for example, a scintillation screen or crystal and photo multiplier or photo diode, so that for each sweep of the pencil beam across the detector, an electrical signal is produced. The electrical signal, as a function of time, represents the intensity of the impinging radiation and hence, the density of the object being illuminated, or that portion of the object illuminated during that particular sweep. These electrical signals can be used to directly generate a video image or can be stored, for example, by sampling, A/D converting and storage in a computer system. The computer system (which may comprise a mini-computer or microprocessor) is advantageous since it can retain the signals for convenient display at any time, and can also provide for processing of the signal so as to provide for various types of image enhancement.
A perennial problem in all types of radiant energy imaging is a desire to maximize resolution and contrast. Typically, it is difficult to achieve both simultaneously. In the pencil beam type systems these competing considerations require a choice to be made respecting the cross-section of the pencil beam (which typically establishes resolution) and amount of the detected X-ray flux (which typically establishes contrast). Decreasing the size of the pencil beam increases the resolution but decreases the contrast. Resolution seems to be achieveable only at the direct expense of contrast, and vice versa. Examples of prior suggestions relating to the relationship between contrast, resolution and pencil beam cross-section are found in Annis et al Ser. No. 900,380 filed Apr. 26, 1978 (as a C.I.P. of Ser. No. 782,972, filed Mar. 30, 1977) and Annis Ser. No. 946,913 filed Sept. 28, 1978 (as a continuation of Ser. No. 782,973, filed Mar. 30, 1977). In both these applications a scanning pencil beam imaging device is disclosed which can develop an image using one of plural possible pencil beam cross-sections. In other types of radiant energy imaging, these competing considerations take different forms, however, maintaining dosage reasonably constant, one can only maximize resolution at the expense of contrast, and vice versa.
It is a specific object of the present invention to improve radiant energy imaging systems which employ a scanning pencil beam of penetrating radiant energy to improve the resolution without significantly sacrificing contrast and/or improve contrast without significantly sacrificing resolution. It is another object of the invention to provide a system which meets the foregoing objects of the invention and which has utility as well in computed tomography imaging systems.